Our invention relates to analog to digital (A-D) conversion systems, and more specifically to a system for digitizing an audio or like analog information signal, with the addition of analog dither (artificially created white noise signal) to the analog information signal and the subsequent removal of the dither from the digitized information signal for the reduction of noise and distortion. Still more specifically, our invention deals with such A-D conversion systems suitable for use in digital audio tape (DAT) systems, among other applications.
In the pulse code modulation (PCM) or digital processing of audio signals, one of the problems that must be overcome is so called "quantization noise", that is, the noise arising from the differences between the samples of the music wave and the quantized values of the samples. The quantization noise becomes particularly pronounced, distorting the input audio signal as higher harmonics, when the input signal is of low amplitude and so requires a relatively small number of quantization steps. Even when the input signal level is comparatively high, the quantization noise will manifest itself if the signal level changes very slowly. Additionally, the analog to digital converter (ADC) itself, which is incorporated in the A-D conversion system, tends to introduce nonlinear distortion during the digitization of the audio signal.
We are aware that dither has been used conventionally for the solution of the above discussed problems. U.S. Pat. No. 4,751,496, issued to Araki et al. and assigned to the assignee of our present application, discloses an example of A-D conversion system utilizing dither, with emphasis on how to prevent the overloading of the ADC when the audio signal to be digitized is of large amplitude.
Generally, analog dither is superposed on the audio signal prior to its A-D conversion and, following the conversion, is removed from the digitized audio signal. The removal of the dither component from the digitized audio signal requires the preparation of digital dither equivalent to the analog dither that has been added to the audio signal. The dither component is removed as the digitized audio signal and the digital dither are both directed into a subtracter.